1. Field of the Invention
The invention relates to a fuel injection system for an internal combustion engine, having a control unit, having a high-pressure fuel pump with at least one pump element, and having a prefeed pump, wherein the prefeed pump pumps fuel from a tank to the intake side of the pump element or elements, and the fuel quantity pumped by the prefeed pump is carried to the pump element or elements or additionally to a low-pressure region of the high-pressure fuel pump.
2. Description of the Prior Art
A high-pressure fuel pump of the above type, known from German Patent Disclosure DE 196 53 339 A1, has a multiposition valve, which as a function of the pressure on the intake side of the high-pressure fuel pump distributes the fuel, pumped by the prefeed pump, to the low-pressure region of the high-pressure fuel pump or to the intake side of the pump elements. With this multiposition valve, there is a need to prevent resultant damage to the high-pressure fuel pump, such as might happen if because of a plugged fuel filter the pumping quantity of the prefeed pump should be inadequate. Through a throttle bore in the multiposition valve, the low-pressure region of the high-pressure fuel pump is in permanent communication with the intake side of the high-pressure fuel pump. The pumping capacity of the electric prefeed pump must therefore be dimensioned as relatively great. In this high-pressure fuel pump, no zero-feed throttle is provided. Instead, the pressure regulation is effected on the high-pressure side of the high-pressure fuel pump by means of a pressure limiting valve, which leads to poor overall efficiency of the fuel injection system.
It is the object of the invention to furnish a fuel injection system whose operating behavior in most operating states is improved, whose construction is simplified, and which is adaptable in the simplest possible way to various internal combustion engines and prefeed pumps.
According to the invention, for a fuel injection system for an internal combustion engine, having a control unit, having a high-pressure fuel pump with at least one pump element, and having a prefeed pump, wherein the prefeed pump pumps fuel from a tank to the intake side of the pump element or elements, this object is attained in that the fuel quantity pumped by the prefeed pump is carried to the pump element or elements or additionally to a low-pressure region of the high-pressure fuel pump by a continuously variable multiposition valve, and that the multiposition valve is triggered by the control unit as a function of the operating state of the engine and/or of the fuel injection system.
By the use of a continuously adjustable multiposition valve, the pumping quantity regulation of the high-pressure fuel pump can be done more efficiently, since the fuel quantity aspirated by the high-pressure fuel pump is regulated, and the fuel that is at high pressure is not throttled for the sake of pressure regulation on the compression side of the high-pressure fuel pump. Moreover, by means of suitable triggering of the continuously adjustable multiposition valve, upon starting of the engine the entire pumping quantity of the prefeed pump can be carried to the pump elements, so that the pressure buildup on the compression side of the high-pressure fuel pump takes place as fast as possible, and thus the engine also starts as fast as possible. Moreover, by means of suitable triggering of the continuously adjustable multiposition valve, ventilation of the high-pressure fuel pump can be performed, should that be necessary. Furthermore, both the cooling and the lubrication of the high-pressure fuel pump can be effected by suitable triggering of the continuously adjustable multiposition valve in accordance with the operating states of the high-pressure fuel pump and/or of the engine, so that adequate lubrication and cooling of the high-pressure fuel pump is always assured.
In a variant of the high-pressure fuel pump employed in the system of the invention, it is provided that the multiposition valve is embodied as a single-stage three-way valve, with one inlet and with a first outlet and a second outlet; that the inlet of the multiposition valve is in communication with the compression side of the prefeed pump; that the first outlet of the multiposition valve is in communication with the intake side of the pump element or elements; and that the second outlet of the multiposition valve is in communication with the low-pressure region of the high-pressure fuel pump. In this variant, the advantages of the invention can be realized in a simple way. In particular, depending on the triggering of the multiposition valve, the entire pumping quantity of the prefeed pump can be carried to the pump elements; the low-pressure region of the high-pressure fuel pump can be ventilated, and both cooling and lubrication of the high-pressure fuel pump can be dimensioned to suit the need. Moreover, the prefeed pump must pump only counter to the requisite minimum pressure, so that the energy consumption of the prefeed pump is minimized and its service life is maximized.
In a further feature of the invention, the multiposition valve has a slide guided in a valve housing; in still another feature of the invention, it can be provided that the valve housing has a recess; that the inlet of the multiposition valve discharges into the recess; and that the slide, as a function of its position in the valve housing, uncovers the first outlet and the second outlet of the multiposition valve to a greater or lesser extent.
In a further feature of the invention, it can be provided that the multiposition valve uncovers the first outlet and blocks the second outlet, if there is no input signal at the multiposition valve, and/or that with an increasing input signal, the slide increasingly uncovers the second outlet and increasingly blocks the first outlet, so that in a simple way, the desired distribution of the fuel stream pumped by the prefeed pump can be accomplished to the pump elements and/or to the low-pressure region.
In another embodiment of the invention, a valve seat cooperating with the slide is embodied in the valve housing and is closed by the slide when the multiposition valve is fully triggered and thus disconnects the first outlet from the inlet, so that no further fuel reaches the pump elements. This valve position is advantageous above all in the overrunning mode of the engine. Because of the low pressures that act on the multiposition valve of the invention, this valve seat can seal off the first outlet reliably and without leakage, so that the pump elements do not aspirate any fuel, and thus a zero-feed throttle in the high-pressure region of the high-pressure fuel pump can be dispensed with. This further reduces the energy consumption of the prefeed pump and moreover further improves the starting performance of the engine.
To assure a minimum supply of fuel for lubrication and cooling purposes to the low-pressure region of the high-pressure fuel pump, it can be provided that a throttle is present between the first outlet and the second outlet. Alternative embodiments of the invention provide that the throttle is embodied as a bore in the slide or in the valve housing or is embodied by the play between the slide and the valve housing.
To further limit the power consumption of the prefeed pump, it is provided in a further feature of the invention that a pressure limiting valve is provided between the inlet and an intake side of the prefeed pump. This pressure limiting valve is advantageous above all whenever the prefeed pump is driven by the engine or the high-pressure fuel pump, and thus the pumping capacity rises virtually linearly with increasing engine rpm. The pressure limiting valve prevents the power demand of the prefeed pump from also rising in proportion to the engine rpm.
The object referred at the outset is also attained according to the invention by a method for generating and delivering fuel, which is at high pressure, in a fuel injection system, having a control unit, having at least one pump element, and having a prefeed pump, wherein the prefeed pump pumps fuel from a tank to the intake side of the pump element or elements, and the fuel quantity pumped by the prefeed pump is additionally carried by a multiposition valve to the pump element or elements or additionally to a low-pressure region of the high-pressure fuel pump, as a result of the provision that the pumping quantity of the prefeed pump is distributed, as a function of the operating state of the engine and/or of the fuel injection system, to the intake side of the pump element or elements and to the low-pressure region of the high-pressure fuel pump. With the method of the invention, the advantages of the invention discussed above can also be achieved.